I have a party of 2nd level characters and I have tried this battle six or seven times and it is total party kill without taking out even one bad guy every time. Given that the fight comes so early in the game, it seems very overpowered. Did I miss a bunch of stuff that would help be higher level before this fight?
Using summon spells and Magic Missile is very good advice. A Cleric or Bishop with any of the three summoning Domain Powers (Justice, Aberration and Undeath) would really help.
You can also go all out with your party's abilities and available magic items.
Use the Drake's breath weapon, use Turn Undead to deal damage, activate a Bard song like Battle March, use Bard spells like Pipes of Pain to deal damage or Zone of Silence to stop the Shadow Cleric from summoning, activate the Barbarian's Rage and Spirit Animals, and make use of any magic scrolls and magic acorns that you may be carrying.
Using Ready Versus Spell on the Shadow Cleric may also be a good idea.
That fight is one out of three possible night-time encounters, so you could also just skip that one and focus on the other two encounters.
Thank you for posting vakil67 and Volourn! Best regards :-)